The present invention is directed to the isolation of active devices on a silicon chip, and more particularly to the complete isolation of islands of silicon from a substrate material.
As active devices, e.g. transistors, become more closely packed together on a semiconductor chip, it becomes necessary to isolate them from one another so that they do not adversely affect each other's operation. For example, in CMOS technology, it is desirable to isolate the well in which the NMOS transistor is typically formed from the substrate which contains the PMOS transistor, so as to diminish the likelihood of latch-up. In the past, lateral isolation of devices from one another has been accomplished by forming a trench around the island of semiconductor material to be isolated and filling the trench with an insulating material, such as oxide. With this approach, the horizontal flow of charge carriers within the chip is prevented so as to reduce latch-up or similar such adverse conditions.
However, the island is not completely isolated in that it remains in contact with the semiconductor substrate. As the size of active devices becomes increasingly smaller, with a concomitant reduction in the amplitude of the signal conducted thereby, this communication between the active device and the substrate can lead to significant adverse effects. For example, the capacitance between the device and the substrate might be large enough to affect the small amplitude signal conducted by the device. Another factor that takes on increasing importance is ambient radiation. More particularly, alpha particles generate charges when they enter a semiconductor chip. The number of charges is related to the volume of semiconductor material in which the charges are generated. Thus, since the substrate occupies a much larger volume than any of the individual active areas, most of the charges are generated in this portion of the chip.
Accordingly, it is desirable to be able to completely isolate an active island not only laterally from adjacent active areas but also vertically from the substrate on which it is supported. In essence, such isolation involves the formation of an insulating layer beneath the active region as well as around it. In the past, three basic approaches have been suggested for providing an active silicon region on an insulating layer. One of these approaches is the formation of a buried silicon dioxide layer through the implantation of oxygen molecules into the silicon substrate. This technique requires a very heavy oxygen dose which can result in significant damage to the active silicon region overlying the buried oxide layer.
A second approach has been to form the silicon on a sapphire substrate. The sapphire functions as an insulator to isolate the active silicon area from similar such areas. However, the thin silicon film typically contains a high concentration of crystalline defects which are produced during an epitaxial growth process. These defects result in lower carrier mobilities than can be achieved in substrate silicon or in epitaxial silicon films grown on a silicon substrate.
Another suggested approach has been to form V-shaped grooves around the regions to be isolated. These grooves are filled with an oxide and a layer of polysilicon is deposited over the oxide. This structure is then inverted and the silicon which originally formed the bottom of the structure is etched back until the apex of each oxide-filled groove appears. The resulting structure is comprised of regions of silicon which are each completely surrounded by oxide. While this structure has good electrical and mechanical properties, the technique for making it is not production-oriented. More particularly, the etching process must be observed very carefully in order that it be stopped at precisely the right time. As a result, a structure produced by this technique can be relatively expensive and is therefore only practically suited for custom integrated circuits. The V-groove isolation technique is not acceptable for the large scale production of memory or logic devices.